The present invention relates to a method for producing a circumferentially closed hollow profile and to a device for performing the method.
A generic method and a generic device are described in European Published Patent Application No. 0 913 277. A wishbone of a wheel suspension may be gathered from this, the wishbone being manufactured from a tube by internal high-pressure forming. The wishbone has various cross-sectional shapes, one of these being a flattened rectangle. In order to produce this shape, the circular-cylindrical prebent tube is introduced into an internal high-pressure forming tool divided into two and, during the closing operation, is squeezed together by the two die parts. Subsequently, with the tool closed, the tube is expanded by internal high pressure, until it is to come to bear exactly to contour against the die impression, and therefore the desired flat rectangular final shape is to be obtained. During squeezing actions of this type, which cause folds, in conjunction with volume-enlarging expansions by internal high pressure, however, a failure of the material often occurs, this being due to an appreciable extent to the strain-hardening of the material achieved after the squeezing operation or to the excessive partial ironings of material in the regions which have not yet come to bear. The failure of the material is manifested, in this case, by the tearing or breaking of the tube or hollow profile. The generally known counterstays cannot be used in this case in order to eliminate this defect, since, on the one hand, the solid counterstays cannot become correspondingly narrower during the squeezing of the tube. On the other hand, the contour of the supporting surface of the plunger is invariable, so that the bearing contact of the tube, whether during the squeezing operation or during the expansion phase, is at no time equally distributed, thus leading to a non-uniform support of the tube and therefore contributing to the failure of the tube at this supported point or in the regions adjacent to the counterstay.
Even a straightforward expansion of a tube of circular cross-section with high degrees of expansion, in which the cross-sectional shape is maintained, does not proceed, when free of support, in a reliable way in terms of the process, since the rate of expansion increases and the tube material would fail when it reached its breaking elongation. In order to counteract this, conventional solid counterstays are used, by which controlled expansion is possible, but limits are also placed on it, since, of course, the tube material comes to bear against the counterstay and experiences there appreciable friction which is detrimental to expansion. Moreover, all-around support by conventional counterstays is virtually impossible during the entire expansion process, thus leading, as described above, to the failure of the tube material in the regions adjacent to the respective counterstay.
It is an object of the present invention to provide a method and a device, to the effect that it becomes possible in the simple manner to perform a production of a hollow profile in which, even in the case of high expansions of the hollow profile (higher than or equal to the breaking elongation of the material), process reliability is ensured to a sufficient extent.
The above and other beneficial objects of the present invention are achieved by providing a method and device as described herein.
In accordance with the present invention, by the diaphragm, a flexible counterstay is formed, which, during expansion forming and also in other forming processes, may adapt to any shape of the hollow-profile blank exactly to contour and in a large area over a relatively large partial circumferential region of the hollow-profile blank. The contour-matching expansion and supporting force of the diaphragm, achieved by the external application of pressure, may be adjusted very accurately to the forming progress by the simple-to-handle pressure control parameters. Overall, that is to say, owing to the large-area bearing contactxe2x80x94circumferentially complete bearing contact if a plurality of diaphragms distributed in the circumferential direction are usedxe2x80x94, during each forming phase and as a result of accurate metering of the supporting force, the hollow-profile blank to be formed receives the appropriate uniform supporting force which prevents a failure of the blank material during expansion. The process reliability of the forming process is thereby ensured, even in the case of very high expansions. What is meant by high expansion is an expansion higher than or equal to the breaking elongation of the material. A diaphragm resistant to high pressure is simple to produce and to fasten and, overall, constitutes only a very low outlay in terms of apparatus. Furthermore, existing forming tools may readily be retrofitted with the diaphragm. Due to the elasticity of the diaphragm, during the interaction of the two oppositely directed pressures of the constant or increasing internal high pressure in the hollow-profile blank and of the pressure, decreasing during expansion, of the external application of pressure to the diaphragm, the latter is withdrawn from the cavity, while maintaining bearing contact which is exact to contour. As a result, as regards the entire forming process, the production of a flattened final shape may thus also occur reliably in terms of the process.
The present invention is explained in more detail below with reference to an example embodiment illustrated in the Figures.